Abstract: We convert propagating qubits encoded as superpositions of zero and one
photons to the motion of a micrometer-sized mechanical resonator. Using quantum
state tomography, we determine the density matrix of both the propagating
photons and the mechanical resonator. By comparing a sufficient set of states
before and after conversion, we determine the average process fidelity to be
$F_{\textrm{avg}} = 0.83\substack{+0.03-0.06}$ which exceeds the classical
bound for the conversion of an arbitrary qubit state. This conversion ability
is necessary for using mechanical resonators in emerging quantum communication
and modular quantum computation architectures.